tests: Add tests for things that are not already tested.

The aim here is to improve coverage of the code.

tests/basics/return1.py

@@ -0,0 +1,13 @@
+# test return statement
+
+def f():
+    return
+print(f())
+
+def g():
+    return 1
+print(g())
+
+def f(x):
+    return 1 if x else 2
+print(f(0), f(1))